This allows callers to (optionally) set a specific limit for audio and
video frames, separately.
Since these are not really stream options, I had to add a new parsing logic
to handle the :a and :v suffixes, but I don't think it's too ugly.
As an aside, also fixes a memleak if setting the option somehow fails.
Signed-off-by: Niklas Haas <git@haasn.dev>